Re: [PATCH v27 23/31] x86/cet/shstk: Add user-mode shadow stack support

["Yu, Yu-cheng" <yu-cheng.yu@xxxxxxxxx>]

On 7/19/2021 11:23 AM, Edgecombe, Rick P wrote:
> On Fri, 2021-05-21 at 15:12 -0700, Yu-cheng Yu wrote:
> > Introduce basic shadow stack enabling/disabling/allocation routines.
> > A task's shadow stack is allocated from memory with VM_SHADOW_STACK
> > flag
> > and has a fixed size of min(RLIMIT_STACK, 4GB).
> >
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>

[...]

> > diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> > new file mode 100644
> > index 000000000000..5ea2b494e9f9
> > --- /dev/null
> > +++ b/arch/x86/kernel/shstk.c

[...]

> > +int shstk_setup(void)
> > +{
> > +       struct thread_shstk *shstk = &current->thread.shstk;
> > +       unsigned long addr, size;
> > +
> > +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> > +               return -EOPNOTSUPP;
> The only caller of this will skip it if
> !cpu_feature_enabled(X86_FEATURE_SHSTK), so this is dead logic. Same
> pattern in the IBT patch.

Indeed that is the case.  We can simply remove the test of 
X86_FEATURE_SHSTK.

Thanks,
Yu-cheng